The invention relates to pressure sensors and, more particularly, touch mode capacitive pressure sensors.
In various industrial and commercial applications, it is desired to measure pressure in a hostile environment, with a miniaturized sensor having good stability, low power consumption, robust structure, large over pressure protection range, and good linearity and sensitivity. For example, such a sensor could be used in conjunction with an RF transponder disposed within a pneumatic tire as shown in commonly-owned, copending PCT patent application No. US98/07338 filed Apr. 14, 1998, incorporated in its entirety by reference herein. Applications such as the pneumatic tire place additional requirements on the pressure sensor due to the need for the sensor to withstand both the normal operating temperature and pressure ranges and also the much higher (many times the operating values) manufacturing temperature and pressure. For example, molding the sensor into a tire is but one illustration of an environment where conventional sensors fail to meet these desired criteria.
Capacitive pressure sensors are known, and can be designed to meet many if not all of the desired characteristics. Capacitive pressure sensors generally include two capacitive elements (plates, or electrodes), one of which is typically a thin diaphragm, and a gap between the electrodes. When a pressure is exerted on the diaphragm, the diaphragm deflects (deforms) and the size of the gap (in other words, the distance between the two capacitive elements) varies. And, as the gap varies, the capacitance of the sensor varies. Such changes in capacitance can be manifested, by associated electronic circuitry, as an electronic signal having a characteristic, such as voltage or frequency, indicative of the pressure exerted upon the sensor.
In the xe2x80x9cnormalxe2x80x9d operation mode of a capacitive pressure sensor, the diaphragm does not contact the fixed electrode. The output capacitance is nonlinear due to an inverse relationship between the capacitance and the gap which is a function of pressure P. This nonlinearity becomes significant for large deflections. Many efforts have been made to reduce the nonlinear characteristics of capacitive sensors either by modifying the structure of the sensors or by using special non-linear converter circuits.
Particularly advantageous for the achievement of linearity has been the development of xe2x80x9ctouch mode capacitive pressure sensorsxe2x80x9d (TMCPS). A particular class of capacitive pressure sensors operate in what is known as xe2x80x9ctouch modexe2x80x9d. Touch mode sensors have been disclosed, for example, in Ding, et al., Touch Mode Silicon Capacitive Pressure Sensors, 1990 ASME Winter Annual Meeting, Nov. 25, 1990, incorporated in its entirety by reference herein. They are further explained in Ko and Wang, Touch Mode Capacitive Pressure Sensors, Sensors and Actuators 2303 (1999), also incorporated in its entirety by reference herein.
Conventional capacitive pressure sensors normally operate in a pressure range where the diaphragm is kept from contacting the underlying electrode, and normally exhibit nonlinear characteristics. This inherent non-linearity has led to the development of many linearization schemes using complex and costly interface circuits which include analog circuits and amplifiers, segment linearization, microprocessor and ROM matrix linearization, etc.
In contrast thereto, touch mode capacitive pressure sensors, operating in the range where the diaphragm touches the insulating layer on the underlying electrode, exhibit near linear behavior in certain pressure ranges. The increased linearity is attributable to the touched area (footprint) increasing linearly with applied pressure and the increased rigidity of the diaphragm after touch.
Furthermore, the touch mode capacitive pressure sensor has much higher sensitivity (large capacitance change per unit pressure change) compared to conventional capacitive pressure sensors. Therefore, small environmentally-caused capacitance changes over time become insignificant and can be neglected. This makes the touch mode device a long term stable device over a wide range of environmental conditions.
These advantages are inherent with touch mode capacitive devices, no matter what materials are used for the diaphragm and the substrate.
Generally, touch mode capacitive pressure sensors differ from conventional capacitive pressure sensors (described hereinabove) in that the diaphragm element is permitted to deflect sufficiently to come into actual physical contact with the underlying fixed capacitive element at a given pressure. Typically, a thin dielectric insulating layer on the fixed capacitive element prevents the diaphragm from electrically shorting to the fixed capacitive element. As the pressure increases, the xe2x80x9cfootprintxe2x80x9d of the diaphragm upon the fixed capacitive element increases, thereby altering the capacitance of the sensor, which can be manifested, by associated electronic circuitry, as an electronic signal having a characteristic indicative of the pressure exerted upon the sensor. The major component of the touch mode sensor capacitance is that of the touched area footprint where the effective gap is the thickness of the thin insulator layer between the pressed-together capacitive elements. Because of the small thickness and large dielectric constant of the isolation layer, the capacitance per unit area is much larger than that of the untouched area which still has an added air or vacuum gap. In a certain pressure range, the touched area is nearly proportional to the applied pressure, and results in the nearly linear C-P (capacitance-pressure) characteristics of the touch mode pressure sensor. For the range of pressures in the touch mode operation region, the sensor capacitance varies with pressure nearly linearly and the sensitivity (dC/dP) is much larger than that in the near linear region of a normal mode device. In addition to high sensitivity and good linearity, the fixed element substrate provides support to the diaphragm after it touches, thus enabling the device to have significant pressure over-load protection. In summary, the advantages of TMCPS are nearly linear C-P characteristics, large overload protection, high sensitivity and simple robust structure that can withstand industrial handling and harsh environments.
As used herein, a xe2x80x9ctouch modexe2x80x9d capacitive pressure sensor includes any capacitive pressure sensor wherein at least a portion of the operating range of the pressure sensor occurs while the diaphragm is in physical contact with the underlying capacitive element.
An early example of a capacitive touch mode pressure sensor is shown in U.S. Pat. No. 3,993,939, entitled PRESSURE VARIABLE CAPACITOR, incorporated in its entirety by reference herein. This patent discloses a large scale version of TMCPS with a variety of diaphragm constructions.
Some of the more recent efforts have focused on miniaturization, cost reduction, and performance improvements. An example is shown in U.S. Pat. No. 5,528,452, entitled CAPACITIVE ABSOLUTE PRESSURE SENSOR, incorporated in its entirety by reference herein. This patent discloses a sensor comprising a substrate having an electrode deposited thereon and a diaphragm assembly disposed on the substrate.
The TMCPS diaphragm can be made of different materials, such as silicon, poly-silicon, silicon nitride, polymeric materials, metal, and metallized ceramic. Each material choice has its advantages and disadvantages. For good stability, robust structure, and avoidance of temperature and pressure related problems, the use of single crystal silicon is preferred, because it has well characterized, well understood, reliable and reproducible electrical and mechanical properties. The aforementioned U.S. Pat. No. 5,528,452 discloses a preferred embodiment with a single crystal silicon diaphragm, which is electrostatically bonded (anodic bonding) to a glass substrate. Although this provides a simple construction which avoids prior art problems with sealing of the vacuum chamber contained between the capacitive elements, it still requires special techniques to fill the gap around the electrical feedthrough from the fixed electrode inside the vacuum chamber to the external electrical connection. The fixed electrode and the connected feedthrough consist of a thin layer of metal deposited on the surface of the glass substrate, covered by an insulating layer. The feedthrough creates a raised line which must pass under the sealing edge of the silicon diaphragm assembly. In order to seal around the feedthrough, a groove is cut in the sealing edge of the silicon diaphragm assembly. With proper shaping of the feedthrough and groove, and with proper alignment, a suitable heat treatment will cause the glass insulating layer to deform and seal the space around the feedthrough in the diaphragm assembly groove.
All-silicon capacitive pressure sensors are characterized by high pressure sensitivity, low mechanical interference and low temperature sensitivity. They can operate up to a temperature of about 300xc2x0 C., and remain almost free of hysteresis.
Drift exhibited by silicon-to-glass capacitive pressure sensors is believed to be caused by mismatch between the thermal expansion coefficients of the glass and the silicon, and the stress built during fabrication. Other problems can arise under temperature extremes which may cause the glass or glass frit to outgas. Using silicon-to-silicon fusion bonding technology, a single crystal capacitive pressure sensor can be realized. For silicon-to-silicon bonded structures, both wafers have the same thermal expansion coefficients, and a better thermal stability is expected than with the silicon-to-glass structure. Using the silicon fusion bonding method, an ultra-stable, high temperature, capacitive pressure sensor can be made.
Silicon fusion bonding is a known technique for bonding together silicon components, either directly silicon-to-silicon, or via an intermediary silicon oxide layer. The latter technique is disclosed, for example, in U.S. Pat. No. 3,288,656, entitles SEMICONDUCTOR DEVICE, incorporated in its entirety by reference herein. A limitation of this technique is that the surfaces to be bonded must be microscopically smooth in order to achieve the intimate contact needed to form good silicon-to-silicon or silicon-to-silicon oxide fusion bonds.
An example of a capacitive pressure sensor with an all-silicon vacuum chamber fabricated using silicon fusion bonding can be seen in U.S. Pat. No. 5,656,781, entitled CAPACITIVE PRESSURE TRANSDUCER STRUCTURE WITH A SEALED VACUUM CHAMBER FORMED BY TWO BONDED SILICON WAFERS, incorporated in its entirety by reference herein. This sensor avoids the problems of sealing around a feedthrough by placing the fixed electrode on a substrate adjacent to the silicon diaphragm but outside the silicon vacuum chamber. As such, the sensor cannot function in a touch mode since ambient pressure deflects the diaphragm away from the fixed electrode.
Another example of an all-silicon, touch mode capacitive pressure sensor is shown in U.S. Pat. No. 5,706,565, entitled METHOD FOR MAKING AN ALL-SILICON CAPACITIVE PRESSURE SENSOR, incorporated in its entirety by reference herein. This patent discloses a sensor with a vacuum chamber formed as a cavity etched into a single crystal silicon wafer doped to be conductive, has an insulating oxide layer on top of the wafer, and a conductive heavily-doped (P+) single crystal silicon diaphragm which is bonded on top of the oxide layer. Among the problems inherent in this design are the expense of a relatively complicated process and xe2x80x9cstray capacitancexe2x80x9d effects. The patent refers to xe2x80x9cconventional wafer-to-wafer bonding techniquesxe2x80x9d for bonding the diaphragm to the substrate. The conventional wafer-to-wafer bonding technique known to those practiced in the art is silicon fusion bonding, which requires microscopically smooth bonding surfaces, as mentioned hereinabove. In order to achieve such a smooth surface on a heavily doped silicon wafer, the conventional technique uses ion bombardment for the P+ doping, rather than diffusion, because the diffusion process roughens the silicon wafer surface. Unfortunately, ion bombardment doping is much more time consuming, and therefore more expensive, than diffusion doping.
The term xe2x80x9cstray capacitancexe2x80x9d refers to capacitance other than the pressure-sensing capacitance, occurring elsewhere in the sensor or its measuring circuit. In the device of the U.S. Pat. No. 5,706,565 mentioned hereinabove, the sensing capacitance occurs between the conductive diaphragm and the fixed electrode (the conductive substrate at the bottom of the cavity), which are separated by a thin dielectric layer. The conductive silicon diaphragm is bonded to an insulating oxide layer on top of the conductive silicon substrate and this forms a second capacitor in parallel with the sensing capacitor. The second capacitor provides stray capacitance with a value which is mainly determined by the oxide layer characteristics and by the total area of contact between the diaphragm and the oxide layer. In order to achieve a good bond, this surface area is typically large enough to produce stray capacitance values more than an order of magnitude larger than the pressure sensing capacitance values.
Problems with capacitive pressure sensors can arise when the sensor must operate over a wide range of temperatures and over a wide range of pressures. Moreover, in some instances, it is essential that the pressure sensor simply survive temporary extremely hostile temperatures and/or pressures, even though it is not operating. For example, molding a pressure sensor into a tire during the fabrication of the tire is an example of a situation where the pressure sensor must survive extremely hostile ambient conditions.
An absolute pressure sensor should also preferably be stable, and free from base line drift problems, both short term and long term. These characteristics are particularly important for applications such as sensors disposed inside tires, where ambient conditions vary significantly and access for maintenance and calibration is limited and relatively costly.
It is an object of this invention to provide a force sensor and a manufacturing method for a force sensor (such as a capacitive pressure sensor) as defined in one or more of the appended claims and, as such, having the capability of being constructed and manufactured to accomplish one or more of the following subsidiary objects.
It is an object of this invention to provide a force sensor and a manufacturing method for a force sensor (such as a capacitive pressure sensor) which has long term stability, and good linear sensitivity, even in harsh environments, both in operation and in OEM application of the sensor in manufactured goods, e.g., such as building the sensor into the rubber of a pneumatic tire, and utilizing the sensor during operation of the tire.
It is an object that the sensor be rugged enough to survive and operate accurately and reliably in these harsh conditions.
It is a further object of this invention to provide a force sensor and a manufacturing method for a force sensor (such as a capacitive pressure sensor) which maintains a hermetic seal in the sensing cavity, while providing an isolated, buried electrical feedthrough to the fixed electrode in the sensing cavity.
It is a further object of this invention to provide a force sensor and a manufacturing method for a force sensor (such as a capacitive pressure sensor made with a silicon substrate and diaphragm) which is cost reduced by providing a means for silicon fusion bonding two silicon wafer surfaces, at least one of which is heavily doped by diffusion.
It is a further object of this invention to provide a force sensor and a manufacturing method for a force sensor (such as a capacitive pressure sensor made with a silicon substrate and diaphragm) which improves sensor stability and accuracy by minimizing stray capacitance effects, and by providing a way for interface circuits to utilize an on-chip reference capacitor to minimize ambient temperature effects on sensor sensitivity.
According to the invention, a first method of fabricating silicon capacitive sensors, comprises providing a first silicon wafer and a second silicon wafer; etching a buried electrical feedthrough groove in the first silicon wafer; etching a sensing cavity and a contact cavity, each cavity connected to an opposing end of the groove; forming a continuous and connected conductive area in the bottoms of the groove and the cavities; forming a P+ conductive diaphragm layer on the second silicon wafer by means of diffusion doping, then preparing the surface of the diaphragm layer for bonding by polishing with a chemical-mechanical polishing (CMP) process; then bonding the first and second wafers together using a silicon fusion bonding (SFB) technique; dissolving the second silicon wafer except for the diaphragm; etching open a window through the diaphragm layer to the fixed electrode contact cavity on the first silicon wafer; and finally dicing the plurality of sensors formed by this process.
As an optional extra step, the first process can include a step of forming an oxide layer over the conductive area in the bottom of the sensing cavity, thereby forming a dielectric layer to allow touch mode operation of the capacitive sensor formed by this method. In the same step, an insulating layer of oxide would also be formed over the conductive area in the bottom of the groove and extending into the sensing cavity to provide a continuous insulating layer over the conductive areas in the cavity and buried feedthrough groove.
Another optional step seals the buried feedthrough groove by depositing LTO (low temperature oxide) over the groove exit to the contact window. Doing this in a vacuum creates a vacuum reference chamber out of the sensing cavity, allowing the sensor to read absolute pressure. Leaving this step out would create a differential pressure sensor. After creating a LTO layer, windows must be opened above the electrical contacts, and optionally over the sensing diaphragm.
Another optional step provides a reference capacitor on the same chip, for reduced temperature sensitivity, by forming the reference capacitor using a conductive area near the surface of the substrate for a reference capacitor bottom electrode, establishing an electrical connection to a reference capacitor bottom electrode contact, using the oxide layer as a fixed-gap reference capacitor dielectric, and the top electrode is the diaphragm layer which will not move with pressure over the reference capacitor area because there is no cavity.
Another optional step provides better electrical connections to the chip by depositing a metal layer over the electrical contacts on the chip.
According to the invention, a second method (simplified from the first method) of fabricating silicon capacitive sensors, comprises providing a first silicon wafer and a second silicon wafer; preparing the first silicon wafer with a thick layer of oxide on one side, with the thickness determined by a designed gap of a sensing cavity; forming the sensing cavity by etching a designed cavity shape completely through the oxide layer; forming a P+ conductive diaphragm layer on the second silicon wafer by means of diffusion doping; then preparing the surface of the diaphragm layer by polishing with a chemical-mechanical polishing (CMP) process before bonding the first and second wafers together using a silicon fusion bonding (SFB) technique; dissolving the second silicon wafer except for the diaphragm layer; etching through the diaphragm layer, stopping at the underlying oxide layer to create a grove around the cavity and extending out to one side of the chip, so that the groove defines the extent of the sensing diaphragm (with a connecting path to a diaphragm contact area), and electrically isolates these areas from a remainder of the diaphragm layer; etching a window through the diaphragm layer and the oxide layer, stopping at the underlying first silicon wafer, to create a fixed electrode contact cavity; and finally dicing the plurality of sensors which are formed by this process.
As an optional extra step for the second method, the first process can include a step of forming an oxide layer over the conductive area in the bottom of the sensing cavity, thereby forming a dielectric layer to allow touch mode operation of the capacitive sensor formed by this method.
Another optional step in the second method uses the area covered by the remainder of the diaphragm layer (i.e., not over the sensing cavity) as an on-chip reference capacitor with the substrate as a bottom electrode (shared with the sensing capacitor), with the solid oxide layer as a fixed dielectric, and with the remainder of the diaphragm layer as a top electrode.
As in the first method, another optional step provides better electrical connections to the chip by depositing a metal layer over the electrical contacts on the chip.
Two techniques are features of the sensors and manufacturing methods of this invention.
The buried feedthrough technique utilized in the first method, described hereinabove, can be applied to any process needing to feed an electrical connection into a sealed silicon cavity. The buried feedthrough consists of a conductor in a shallow groove which is almost filled with an optional covering insulating oxide layer. The gap between the top of the insulator and the second silicon wafer which is bonded over the groove and cavity, can then be sealed with LTO.
The second special technique of this invention is a method for forming a silicon-to-silicon fusion bond (SFB) wherein at least one of the two surfaces to be bonded has been heavily boron-doped by means of diffusion, which is a less-costly way of doping, but creates a rough silicon surface unsuitable for good SFB joints. The technique is to prepare each doped surface for SFB by polishing the surface in a Chemical-Mechanical Polishing (CMP) process.